its just like ryanair - a regular user knows what is in store
look at the hertz/europcar/avis prices still add estimated cost of fuel when filling up.
compare with the likes of goldcar etc, add the inlfated fuel price and hen compare what looks best to you.
Bear in mind herz avis etc are scrutenising cars for scratches etc, whilst the likes of goldcar record etc don't give a monkeys.
If 3 days or less - a no brainer stick with the cheaper car rentals as you get option of full full or money refund on what is left in tank , 4-5 days ok work out the maths as you probably wont use much fuel as you will loose out
a week or 2 weeks again the cheaper full - empty are probably still cheaper regardless the intial rip off on fuel
like i said make a educated guess